Ripping the legs off live crabs and crowding lobsters into seafood market tanks are just two of the many practices that may warrant reassessment, given two new studies that indicate crustaceans feel pain and stress.

The findings add to a growing body of evidence that virtually all animals, including fish, shellfish and insects, can suffer.

Robert Elwood, the lead author of both papers, explained to Discovery News that pain allows an individual to be "aware of the potential tissue damage" while experiencing "a huge negative emotion or motivation that it learns to avoid that situation in the future."

Elwood, a professor in the School of Biological Sciences at The Queen's University in Belfast, and colleague Mirjam Appel studied hermit crabs collected from rock pools in County Down, Northern Ireland. All of the crabs survived the experiments and were later released back into their native habitat.

Elwood and Appel gave small electric shocks to some of the crabs within their shells. When the researchers provided vacant shells, some crabs — but only the ones that had been shocked —left their old shells and entered the new ones, showing stress-related behaviors like grooming of the abdomen or rapping of the abdomen against the empty shell.

Grooming, as for a person licking a burnt finger, "is a protective motor reaction and viewed as a sign of pain in vertebrates," the researchers wrote.

It has been thought that the behavior of crustaceans is mostly reflexive, but the fact that they showed signs of physical distress at the same time they changed a behavior — in this case, moving into another shell — suggest they feel pain as well, according to the researchers.

The research has been accepted for publication in the journal Animal Behavior.

For the second paper, slated for publication in the journal Applied Animal Behavior Science, Elwood, along with Stuart Barr and Lynsey Patterson, outline seven reasons, with supportive findings, they believe crustaceans suffer.

For one thing, they argue, crustaceans possess "a suitable central nervous system and receptors." They learn to avoid a negative stimulus after a potentially painful experience. They also engage in protective reactions, such as limping and rubbing, after being hurt.

Physiological changes, including release of adrenal-like hormones, also occur when pain or stress is suspected. And the animals make future decisions based on past likely painful events.

If crabs are given medicine — anesthetics or analgesics — they appear to feel relieved, showing fewer responses to negative stimuli. And finally, the researchers wrote, crustaceans possess "high cognitive ability and sentience."

Rarely seen creaturesIn the past, some scientists reasoned that since pain and stress are associated with the neocortex in humans, all creatures must have this brain structure in order to experience such feelings. More recent studies, however, suggest that crustacean brains and nervous systems are configured differently. For example, fish, lobsters and octopi all have vision, Elwood said, despite lacking a visual cortex, which allows humans to see.

It was also thought that since many invertebrates cast off damaged appendages, it was not harmful for humans to remove legs, tails and other body parts from live crustaceans. Another study led by Patterson, however, found that when humans twisted off legs from crabs, the stress response was so profound that some individuals later died or could not regenerate the lost appendages.

Chris Sherwin, a senior research fellow in the Clinical Veterinary Science division at the University of Bristol, has also studied pain in invertebrates.

Sherwin told Discovery News, "The question of whether invertebrates experience pain is fundamental to our legislation that protects animals and our behavior, attitude and use of these highly complex organisms."

He said that while the recent studies suggest crustaceans experience "something akin to pain, rather than fixed, reflex responses," additional research is needed.